In general, parts of an automotive exhaust system are classified into a hot part and a cold part according to a temperature of an exhaust gas. The automotive components of the hot parts include an exhaust manifold, a converter, a bellows, etc. These are primarily used at a temperature of 600° C. or more, and thus are required to have excellent high temperature strength, thermal fatigue, high temperature salt corrosion properties, etc. On the other hand, the cold parts are used at a temperature of 400° C. or less, such as a muffler for decreasing the noise of an automotive exhaust gas. Due to condensate corrosion properties resulting from a sulfur (S) component in an automotive fuel, exterior rust/corrosion properties according to the use of a snow removal salt in winter, and the like, materials such as stainless (or STS) 409, 409L, 439, 436L and the like are used for the cold parts of the automotive exhaust system.
Particularly, a material of stainless 409 or 409L is a steel grade which includes Cr at 11%, has C and N stabilized by Ti, prevents sensitization of the weld and has excellent formability. The above-described steel grade is primarily used at a temperature of 700° C. or less, has a certain degree of corrosion resistance with respect to components of condensed water generated in the automotive exhaust system, and thus is a steel grade which is most frequently used.
Meanwhile, stainless 439 has C and N stabilized by Ti, and has about 17% Cr. Further, stainless 436L is steel which is obtained by adding about 1% Mo to stainless 439, and has excellent condensate corrosion properties and corrosion resistance properties.
Meanwhile, the S component in automobile fuel components is concentrated in SO42−, and thus is changed to a highly corrosive sulfuric (H2SO4) atmosphere having a pH of 2 or less. When a stainless 409L material, which is most frequently used, is used for an automotive muffler material used in an area where a large amount of the S components are contained in gasoline components as described above, corrosion caused by condensed water or the like occurs. In this case, there is a problem in that it is difficult for the automotive to meet the warranty of a car manufacturer. Accordingly, the use of a high Cr-based stainless material such as stainless 439, 436L or the like including a Cr component at 17% or more is gradually increasing, currently. However, a resource price of this material is gradually increasing, and thus there is a need for the development of a stainless material which is manufactured by adding other elements instead of adding expensive elements such as Mo or the like to have condensate corrosion properties which are at least equal to a material of stainless 439 or 436L.
Meanwhile, an aluminum-plated stainless steel plate which is manufactured by hot-dip coating a stainless 409L material with aluminum has been developed and used in order to improve exterior rust/corrosion properties attributable to the occurrence of surface oxidation or surface discoloration. However, in general, a passivation film is present on a surface of a stainless steel plate, and the passivation film degrades the wetting properties of hot dipping. Accordingly, there is a problem in that aluminum plating is difficult to be performed without a separate treatment process such as atmosphere control.
The present invention relates to a ferritic stainless steel for automotive exhaust system having excellent corrosion resistance, particularly condensate corrosion resistance and formability, and a method of manufacturing the same. The objective of the present invention is directed to providing the ferritic stainless steel having improved condensate corrosion resistance under the environment of the use of a high sulfur fuel and excellent formability, by adding a small amount of an alloying element of Sn, or Sn and Cu to the ferritic stainless steel, and the method of manufacturing the same.
Further, the objective of the present invention is directed to providing the ferritic stainless steel, which has condensate corrosion resistance at least equal to that of a stainless 439 or 436L material of which corrosion resistance is improved by adding an element such as Mo or the like to an existing 17Cr alloy base, and which may also be manufactured without adding an expensive element such as Mo or the like thereto, and the method of manufacturing the same.
Further, the objective of the present invention is directed to providing the ferritic stainless steel, which is plated with aluminum in order to improve condensate corrosion resistance, formability, and high temperature oxidation resistance properties, and the method of manufacturing the same.